This result admits of an experimental test by means of the photographic registration of stars during a total eclipse of the sun.  The only reason why we must wait for a total eclipse is because at every other time the atmosphere is so strongly illuminated by the light from the sun that the stars situated near the sun’s disc are invisible.  The predicted effect can be seen clearly from the accompanying diagram.  If the sun (S) were not present, a star which is practically infinitely distant would be seen in the direction D[1], as observed front the earth.  But as a consequence of the deflection of light from the star by the sun, the star will be seen in the direction D[2], i.e. at a somewhat greater distance from the centre of the sun than corresponds to its real position.

In practice, the question is tested in the following way.  The stars in the neighbourhood of the sun are photographed during a solar eclipse.  In addition, a second photograph of the same stars is taken when the sun is situated at another position in the sky, i.e. a few months earlier or later.  As compared whh the standard photograph, the positions of the stars on the eclipse-photograph ought to appear displaced radially outwards (away from the centre of the sun) by an amount corresponding to the angle a.

We are indebted to the [British] Royal Society and to the Royal Astronomical Society for the investigation of this important deduction.  Undaunted by the [first world] war and by difficulties of both a material and a psychological nature aroused by the war, these societies equipped two expeditions —­ to Sobral (Brazil), and to the island of Principe (West Africa) —­ and sent several of Britain’s most celebrated astronomers (Eddington, Cottingham, Crommelin, Davidson), in order to obtain photographs of the solar eclipse of 29th May, 1919.  The relative discrepancies to be expected between the stellar photographs obtained during the eclipse and the comparison photographs amounted to a few hundredths of a millimetre only.  Thus great accuracy was necessary in making the adjustments required for the taking of the photographs, and in their subsequent measurement.

The results of the measurements confirmed the theory in a thoroughly satisfactory manner.  The rectangular components of the observed and of the calculated deviations of the stars (in seconds of arc) are set forth in the following table of results : 

Table 01:  file table01.gif

(c) Displacement of Spectral Lines Towards the Red

In Section 23 it has been shown that in a system K1 which is in rotation with regard to a Galileian system K, clocks of identical construction, and which are considered at rest with respect to the rotating reference-body, go at rates which are dependent on the positions of the clocks.  We shall now examine this dependence quantitatively.  A clock, which is situated at a distance r from the centre of the disc, has a velocity relative to K which is given by